Extending the capabilities of the Hygroscopic Tandem Differential Mobility Analyzer (HT-DMA)

 
Poster PDF

Authors

Janek Uin — Brookhaven National Laboratory
Gunnar I. Senum — Brookhaven National Laboratory
Stephen R. Springston — Brookhaven National Laboratory

Category

ARM infrastructure

Description

The ARM Hygroscopic Tandem Differential Mobility Analyzer (Brechtel Manufacturing Inc.) measures the change in ambient aerosol particle size (growth factor) due to water uptake, thus providing information about the chemical composition of the aerosol particles. The instrument is part of ARM Aerosol Observing System (AOS) core measurements and has been successfully deployed for several years in various environments during ARM field campaigns. The HT-DMA operates by drying the aerosol sample and then size selecting a narrow range to be humidified. The resulting size distribution is then measured, and using the initial dry particle size, a growth factor is derived. Currently, the instrument operates at a fixed relative humidity and scans through several dry selection sizes to obtain particle growth factors for aerosol particles with different sizes (and possibly different origins). However, the instrument is also capable of scanning through multiple relative humidity set points. If configured to do so, efflorescence and deliquescence scans at specified dry particle size could be obtained. From there, the hygroscopicity parameter kappa could be derived. Furthermore, due to the design of the HT-DMA, the first "dry" DMA can be used as a standalone particle sizer if an additional external CPC were added to the system. This would allow scans of the ambient aerosol size distribution to be made, similar to an SMPS, interspersed with regular HT-DMA growth factor measurements. The benefit in using the HT-DMA for ambient particle size scans over the regular SMPS would be the higher size range (up to 1000 nm) as compared to the already deployed TSI SMPS (limited below 400 nm). Changing the instrument configuration to provide efflorescence and deliquescence scans or ambient SMPS scans would impact the currently made growth factor measurements by either decreasing the time resolution or providing the growth factor only at a fixed dry particle size. Thus, any change would have to be based on user needs and specifics of each deployment to focus on key areas of interest.